Phosphonium salts may be used in a wide range of applications, including: as surfactants; as a component in spinning finish agents for aromatic polyamide fibers (JP11172577 A2, published Jun. 29, 1999); as antimicrobial agents (Kanazawa et al. (1994) Antimicrobial Agents and Chemotherapy, vol. 38(5), p. 945-952); and as polar solvents known as “ionic liquids” (for a recent review, see Thomas Welton (Chem. Rev. 1999, 99, 2071-2083)).
For many purposes, a phosphonium salt having a non-halide anion is desirable. Phosphonium non-halide salts can be prepared by a conventional two-step process, comprising the steps of (a) reacting a tertiary phosphine with an alkylhalide to obtain a quaternary phosphonium halide salt, and (b) exchanging the halide anion with a suitable anion (by ion exchange or metathesis) to generate a quaternary phosphonium salt having a non-halide anion.
However, this two-step process has several drawbacks. For example, the alkylhalides used to quaternize the tertiary phosphine are expensive and some are corrosive and difficult to prepare and use. Also, the tertiary phosphine may be expensive or difficult to make, the preparation thereof sometimes involving several steps and starting materials that are expensive and pyrophoric (see for example Kanazawa et al., supra, and Hugh R. Hays, J. Org. Chem. Vol. 31, pp. 3817-3820, which describe methods for preparing trimethylalkylphosphonium halides).
In addition, the two-step process generates large amounts of waste, as salt or acid by-products are usually removed by washing with water. Thus, the two-step process is inconvenient on an industrial scale.
Further, the end-product of the two-step process can be contaminated with residual halide ion, which may interfere with the intended utility of the phosphonium salt. For instance, halide ions such as chloride ions coordinate with group VII metals such as palladium and platinum and as a result, the presence of chloride ion can interfere with the activity of group VII metal catalysts. If a phosphonium salt is to be used in an environment where halide ions are unacceptable, even at low levels, halide salts should not be used in the starting materials or a further process must be used which ensures removal of halide ions from the phosphonium salt.